In many drilling applications it may be necessary to stop the drilling process before or shortly after the bit penetrates a new formation. In order to determine the position for stopping the drilling industry uses resistivity tools which are sensitive at or near the bit. Those resistivity tools are normally based on the usage of a toroidal coil for transmitting a current along the drill string and a receiver toroidal coil for measuring the current near the bit in direction of the borehole. Typically, a transmitter toroid and a receiver toroid are used, and the receiver toroid is typically located between the bit and the transmitter toroid. The transmitter toroid generates the current along the drill string while the receiver toroid delivers a measured current at a fixed position within the drill string.
These kinds of tools typically provide only one measurement curve. Small changes in resistivity caused by geological noise and/or small resistivity contrast at a boundary between formations may prevent estimation of some parameters ahead of the bit. Also, conditions like temperature may influence the response of the absolute measured resistivity value. The look ahead of the bit capability may be limited in quality since only small changes in resistivity are usually seen by the tool and the above mentioned factors often render such small changes as not interpretable. The information provided by those tools typically includes only one resistivity curve over the depth provided. If a zone of interest is touched, the resistivity values may change. If the contrast between the actual formation and a new formation is not sufficiently high, a misinterpretation of the resistivity curve is possible because of changes in the permeability of the toroidal carrier material to temperature, mechanical stress, etc.